1. Field of the Invention
The present invention relates to a thermal printer which precisely adjusts the rate of paper feed to thereby reduce shifting of colors caused by variations in printing rate, frictional force between the paper and a pinch roller, and variations in the diameter of a platen due to thermal expansion.
2. Prior Art
Description will be given below of the structure of a conventional thermal printer with reference to FIGS. 13 and 14. FIGS. 13 and 14 are views which respectively show the electric circuit and mechanism of the conventional thermal printer.
In FIG. 13, the conventional thermal printer comprises an MPU (a micro processor unit) 1 which controls paper feed and the like, a ROM 2 for storing the data of a program and the like, a RAM 3 for temporarily storing data and the like, an input buffer 4 into which data is input, a printing control circuit 5 including a dot counter and the like, an output buffer 6 for outputting data, an ink motor drive circuit 7, an ink motor 8 for feeding an ink sheet, a platen motor drive circuit 9, a platen motor 10 for driving a platen through a belt or the like, and a bus connecting the above circuits with one another.
Also, as shown in FIG. 14, the conventional thermal printer further includes a feeding roller 14 and a winding roller 15 which pass under a head 12 and feed an ink sheet 13, and a platen 16, a pinch roller 17, and rollers 18 and 19 which form a mechanism to feed paper 21 being held between a clamper 20 and the feed mechanism.
Description will be given of the operation of the above-mentioned conventional thermal printer with reference to FIGS. 15(a) through 15(e), which are timing charts showing the operation of the conventional thermal printer. FIG. 15(a) shows a data receiving synchronizing signal, FIG. 15(b) shows input data, FIG. 15(c) shows data transfer, FIG. 15(d) shows output data, and FIG. 15(e) shows drive pulses of the platen motor.
As shown in FIGS. 15(b) through 15(d), data are input in each of lines into the input buffer 4, are then transferred and are finally output to the head 12 while they are shifted by one line. To the head 12 there are input the information on the printing data and a strobe signal on heat generation.
MPU 1 generates a drive pulse of the platen motor 10 in synchronization with the data receiving synchronizing signal. The number of drive pulses to be generated by an output corresponding to one line is a given number and, in FIG. 15(e), 5 pulses are shown. Also, an exciting pattern is set to the platen motor drive circuit 9. The platen motor drive circuit 9 in turn outputs the exciting pattern to the platen motor 10 in synchronization with the drive pulse. When the platen motor 10 is driven, then the platen 16 is rotated through a belt and the clamper 20 is moved, thereby feeding the paper 21.
The feeding of the paper 21 is started under the head 12 and travels counterclockwise (round the platen 16) for each color, and rollers 18 and 19. When the paper 21 passes under the head 12, the head 12 is pushed up and, when passing between the platen 16 and pinch roller 17, the pinch roller is opened to thereby allow the clamper 20 to go through. In this operation, the ink sheet 13 is being fed by the ink motor 8 at a constant speed equal to that of the paper 21.
However, the above-mentioned conventional thermal printer has a problem that there is color shifting due to unevenness in the paper feeding. Description will be given below of the main causes of the uneven paper feeding. The main causes include a printing rate, frictional force between the printed surface and a pinch roller, and thermal expansion of the platen.
At first, description will be given of the variations in the amount of paper feeding caused by the printing rate with reference to FIGS. 16(a), 16(b) and 17(a) through 17(c). FIG. 16(a) and 16(b) are views showing part of a mechanism of a conventional thermal printer, and FIGS. 17(a) through 17(c) are timing charts showing variations of the paper feeding amount caused by the printing rate.
In particular, FIG. 16(a) illustrates a state of the paper feeding mechanism of the conventional thermal printer in which no dot is printed, whereas FIG. 16(b) illustrates another state thereof in which all dots are printed. The platen 16 is being driven at a constant speed. When printing is not executed, since the ink of the ink sheet 13 is not melted at all, the ink sheet 13 can be easily separated from the paper 21. For this reason, the paper 21 can be drawn with a relatively smaller force.
On the other hand, when printing is executed, since the ink of the ink sheet 13 is melted, it is not easy to separate the ink sheet 13 from the paper 21. This means that the paper 21 must be drawn with a great force. In other words, assuming that a drawing force necessary when no printing is executed is expressed as F.sub.1 and a drawing force for printing is expressed as F.sub.2, then the following relationship holds for them: EQU F.sub.1 &lt;F.sub.2
Although in FIGS. 16(a) and 16(b) there are shown two extreme cases, the drawing force F.sub.2 may vary according to the densities and colors of the paper 21 to be printed.
FIG. 17(a) shows a fire state of the head 12, FIG. 17(b) shows an amount of paper to be fed per unit time, and FIG. 17(c) shows a printing length, respectively, in which the respective abscissas thereof illustrate a printing advancing direction. As shown in FIGS. 17(a) through 17(c), due to the execution of the printing, the drawing force F increases to thereby increase the paper feed amount as well as the printing length. A dotted line shown in FIG. 17(c) represents the variations of the printing length when it is assumed that the paper feed amount is constant. For control of the feed amount, the number of drive pulses of the platen motor 10 during one line printing is set as a given number.
Next, description will be given below of the variations in the paper feed amount caused by a frictional force between a printed surface and a pinch roller with reference to FIGS. 18(a) through 18(d) and 19(a) through 19(e). FIGS. 18(a) through 18(d) are views showing part of a mechanism of a conventional thermal printer, and FIGS. 19(a) through 19(e) are timing charts showing the variations in the paper feed amount caused by a frictional force between the printed surface and pinch roller of the conventional thermal printer.
In particular, FIGS. 18(a) and 18(b) illustrate a case in which something is being printed onto the paper 21, and FIGS. 18(c) and 18(d) illustrate a case in which nothing is being printed onto the paper 21. When printing onto the printed surface, the printed surface provides a greater friction coefficient with respect to the pinch roller 17 because the printed surface is uneven. On the other hand, when printing onto a blank paper surface, the friction coefficient is smaller because the blank paper surface is even. That is, assuming that a friction coefficient between the printed surface and pinch roller 17 is expressed as .mu..sub.1 and a friction coefficient between the blank paper surface and pinch roller 17 is expressed as .mu..sub.2, then the following relationship holds for them: EQU .mu..sub.1 &gt;.mu..sub.2
These friction coefficients have an effect on the printing to be effected on and after the second color.
FIG. 19(a) shows a printing state of paper, FIG. 19(b) shows a friction coefficient between a pinch roller and a printing surface, FIG. 19(c) shows a force to feed paper, FIG. 19(d) shows an amount of feeding of paper, and FIG. 19(e) shows a printing length, in which the respective abscissas thereof represent a printing advancing direction. As shown in FIGS. 19(a) through 19(e), since a friction coefficient with respect to the pinch roller 17 increases due to a printed surface, the force to feed the paper 21 decreases, the amount of paper feeding is reduced, and the printing length is shorter than its normal level. That is, the frictional force of the pinch roller 17 acts as a brake on the paper in the feeding direction thereof. A dotted line shown in FIG. 19(e) represents the variations of the printing length when it is assumed that the amount of paper feed is constant.
Further, description will be given of the variations of the paper feed amount due to the thermal expansion of a platen with reference to FIGS. 20 and 21. FIG. 20 is a view to show a part of a mechanism of a conventional thermal printer, and FIGS. 21(a) through 21(d) are timing charts showing the variations in the paper feed amount due to the thermal expansion of a platen employed in the conventional thermal printer.
As shown in FIG. 20, when a heating element of the head 12 generates heat, then the heat is accumulated in the platen 16 to thereby expand the diameter of the platen 16, with the result that the amount of paper feed per line increases.
FIG. 21(a) shows a rise in temperature of the platen, FIG. 21(b) shows the degree of expansion of the platen, and FIG. 21(c) shows a paper feed amount, and FIG. 21(d) shows a printing length, in which the respective abscissas thereof represent a printing advancing direction. As shown in FIGS. 21(a) through 21(d), when the head 12 is heated, then the temperature of the platen 16 rises to thereby increase the expansion of the platen 16. As a result of this, the diameter of the platen 16 increases, whereby the paper feed amount increases and the printing length increases as well. A dotted line shown in FIG. 21(d) represents variations in the printing length when it is assumed that the paper feed amount is constant.